1. Field of the Invention
The present invention is directed to image forming devices, such as electrostatic copying machines, and in particular to design rules and geometric compensations for image forming devices which prevent misregistration of developed latent images. Such misregistrations arise due to inherent eccentricities in the photoreceptors or the speed reduction drive trains of the image forming devices.
2. Discussion of the Related Art
U.S. Pat. No. 4,531,828 to Hoshino discloses an imaging apparatus which prevents misregistration by designing the driving roller of the screen belt to have a peripheral length equal to an integer multiple of the spacing between the respective transfer stations. As shown in FIG. 1, apparatus 1 comprises four sets of electrophotographic laser beam printer mechanisms I-IV which are substantially identical in construction. Each printer mechanism comprises a photosensitive drum 10 mounted on a rotatable shaft 12, a charging a cleaning device 14, a transfer discharger 16 and a developing device 22.
A laser beam scanner 20 oscillates a laser beam L along the surface of a photosensitive drum 10, which rotates about a shaft 12, and forms a latent image corresponding to an electrical or optical input. Developing devices 22 of printer mechanisms I-IV develop the latent images using yellow (Y), magenta (M), cyan (C), and black (BK) developing toners. Driving rollers 24 and 26 with a diameter D rotate a screen belt 28 to convey a sheet of paper P through transfer stations of printer mechanisms I-IV. The developed image of each printer mechanism is transferred onto the paper to form a multiplexed color image.
Because the driving roller 26 has an inherent eccentricity, resulting from its manufacture or assembly, the peripheral speed of roller 26 will vary from high to low speeds at various angular positions as the screen belt 28 moves past and contacts the roller 26. Thus, the speed imparted to the screen belt 28 is not constant, but rather varies in a sine-wave like fashion. The varying speed imparted to the screen belt 28 causes misregistration of one developed image transferred from one printer mechanism with respect to the other developed images transferred from the other printer mechanism. The misregistration of the developed images causes distortions of the color image on the paper P.
To prevent such misregistration from occurring, the driving roller 26 is designed so that its peripheral length or circumference is equal to be an integer multiple of the spacing H between the respective transfer stations (i.e., the distance travelled by the screen belt between the respective transfer stations, or the image pitch length). Further, the drive roller 26 is driven by a gear train comprised of a combination of forty teeth to twenty teeth so as to provide a reduction gear ratio of 2:1. Thus, even if the gears have inherent eccentricities, the phase angle of the eccentricity of each gear varies identically as the paper passes through each transfer station. Therefore, misregistration is prevented.
U.S. Pat. No. 4,803,515 to Hoshino et al. discloses another source of misregistration caused by an inherent drive non-uniformity of the driving means of the photosensitive drum 10, as shown in FIG. 2. The driving means comprise an electric motor 30, a driving gear 32 and a driver gear 34 and the non-uniformity of the driving means results in the non-uniform rotational speed of photosensitive drum 10. The non-uniform speed of the photosensitive drum 10 expands or shrinks the latent image when the laser beam 20 writes the latent image on the photosensitive drum surface during image exposure steps. As a result, the developed image transferred onto the paper P is expanded or shrunk correspondingly.
In order to solve the problem of the image expansion and shrinkage attributable to the non-uniform drive by the driving means, the photosensitive drum 10 is driven in such a controlled manner that the time required for the photosensitive drum surface to move from an image writing position 10a (latent image forming position) to an image transfer position 10b is an integer multiple of the period of the drive non-uniformity of the photosensitive drum driving means. In other words, the rotational period T1 of a gear 32 is an integral fraction of the time interval T2 of the movement of writing position 10a to transfer position 10b. Because of this arrangement, an integrated pitch error (per one full turn) and an adjacent pitch error (per one tooth), attributable to the gear 32 at the time of image writing, are reproduced at the time of image transfer.
Both of the above patents use a gear train to rotate the roller or the photosensitive drum. A more efficient method of speed reduction is the use of a timing belt speed reduction drive train having two sets of pulleys and timing belts. The timing belt speed reduction drive train is driven by a small, high speed, low torque motor which is less expensive and thus reduces the cost of the image forming device. Further, because the timing belt speed reduction drive train includes numerous pulleys and timing belts, the design criteria of a gear system cannot solve the harmonic motion errors caused by eccentricities in the pulleys and belts. Neither of the above patents discloses design rules to reduce harmonic motion errors caused by eccentricities in the pulleys and the belts.
Further, photosensitive belts can be used for the image forming device rather than photosensitive drums. Neither of the above patents discloses geometric compensations to eliminate registration errors that might otherwise result from harmonic errors (odd or even) in the motion of the photosensitive belts. Moreover, neither of the above U.S. patents discloses photosensitive belts having dimensions which may be chosen independent from the image pitch length.
All references cited in the specification, and their references, are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features, and/or technical background.